This invention relates to a chain tensioner for keeping a constant tension in a chain of a chain transmission device, particularly one for driving camshafts.
One conventional chain tensioner of this type includes a plunger inserted in a cylinder chamber formed in a housing, and a spring for biasing the plunger outwardly of the cylinder chamber. A pressure chamber is defined between the bottom of the housing and the inner end of the plunger. The pressure chamber is filled with hydraulic oil supplied through an oil supply passage formed in the housing to damp the pushing force applied to the plunger.
To assemble such a chain tensioner, the spring is inserted into the cylinder chamber, and the plunger is pushed into the cylinder chamber while compressing the spring. Then, with the plunger kept pushed in the cylinder chamber against the force of the spring, the housing is secured to a stationary member. Since the plunger has to be kept pushed in the cylinder chamber against the force of the spring while assembling the chain tensioner, assembly is troublesome and time-consuming.
In order to solve this problem, an improved chain tensioner was proposed. This tensioner has a pin hole formed in the outer periphery of the plunger near its protruding end and a corresponding pin hole formed through the housing near the open end of the cylinder chamber. In assembling this chain tensioner, with the plunger pushed into the cylinder chamber while compressing the spring until the pin holes align with each other, a pin is inserted into the pin holes to keep the plunger pushed in the housing without the need to continuously apply pushing force to the plunger.
While this chain tensioner can be assembled more easily than the first mentioned chain tensioner, assembly is still not sufficiently easy because the pin has to be pulled out after the chain tensioner has been assembled.
The chain tensioner disclosed in JP utility model publication 58-38189 is free of this problem because the plunger is released under the tension of the chain when the plunger is pushed by the chain.
This chain tensioner has a pin extending from the outer surface of the housing to the inner wall of the cylinder chamber and a pivot plate provided at the front end of the plunger so as to engage the pin. Thus, the pin has to be pressed into the housing so as not to easily fall off the housing.
If the housing is formed of a soft material such as an aluminum alloy, the housing tends to be deformed when the pin is pressed into the hole formed in the housing. Deformation of the housing can cause changes in the dimension of the inner wall of the cylinder chamber and/or strains in the inner wall of the cylinder chamber, which can result in a reduced clearance between the cylinder chamber and the outer periphery of the plunger and thus less smooth sliding of the plunger. This makes it difficult to operate the chain tensioner with high accuracy.
An object of the invention is to provide a chain tensioner having a plunger retaining means which can be automatically released under the tension of a chain and which will not inhibit smooth sliding of the plunger even if the housing is made of a soft material such as an aluminum alloy.